The present invention generally relates to optical displays, and more particularly, is concerned with liquid crystal displays, and even more particularly relates to a method and apparatus for adjusting the liquid crystal transmittance over temperature extremes and ambient light extremes in real time for use in gray scaling for liquid crystal displays.
In today's aviation industry, it is common for a single aircraft to be subjected to several extreme thermal conditions in a relatively short time interval. It is not uncommon for an aircraft to be operated with internal avionics equipment having dynamic temperature ranges of more than 70.degree. C. With the current aspirations for trans-atmospheric aircraft, these extreme vicissitudes in the ambient temperature will continue to confront avionics engineers with perplexing problems of increasing difficulty and importance.
One particular problem that is exacerbated by these temperature oscillations is the continual change of contrast and brightness of liquid crystal displays during take-off and landing of an airplane. Typically, the transmittance versus voltage curve of a particular liquid crystal is a function of liquid crystal temperature and, to some degree, a function of the liquid crystal age. The transmittance versus voltage curve characteristics for liquid crystal is extremely important when grey scaling is attempted. The voltage across the liquid crystal, which is necessary to produce a predetermined level of transmittance of light through the liquid crystal, can vary widely over the typical operating temperature range of a display. This creates a situation where the contrast and brightness of the display are constantly changing, with a concomitant loss of information which is provided by the gray scale.
One method to control the brightness of a display is disclosed in U.S. Pat. No. 4,114,366 to Carl H. Renner et al entitled "Digital Brightness Control", which is incorporated herein by this reference. This brightness control system provides for a light sensor which measures the ambient light intensity and generates an analog signal to regulate a display.
While this method can assist in controlling the brightness of a display it suffers from serious drawbacks. The predominant drawback of such a brightness control system is that the brightness of a display is a function of the sensed ambient light intensity, and there is no control over the varying light transmittance through the liquid crystal at a given voltage as a function of temperature, and therefore does not eliminate the loss of gray scale information.
Consequently, a need exists for an improved method and apparatus for controlling the brightness and contrast of a liquid crystal display over a wide range of temperatures.